This invention relates to wideband code division multiple access (WCDMA) for a communication system and more particularly to Doppler frequency estimation of WCDMA signals with space-time transmit diversity.
Present code division multiple access (CDMA) systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected receiver to determine the proper recipient of a data signal. These different data signals arrive at the receiver via multiple paths due to ground clutter and unpredictable signal reflection. Additive effects of these multiple data signals at the receiver may result in significant fading or variation in received signal strength. In general, this fading due to multiple data paths may be diminished by spreading the transmitted energy over a wide bandwidth. This wide bandwidth results in greatly reduced fading compared to narrow band transmission modes such as frequency division multiple access (FDMA) or time division multiple access (TDMA).
New standards are continually emerging for next generation wideband code division multiple access (WCDMA) communication systems as described in Provisional U.S. Patent Application No. 60/082,671, filed Apr. 22, 1998, and incorporated herein by reference. These WCDMA systems are coherent communications systems with pilot symbol assisted channel estimation schemes. These pilot symbols are transmitted as quadrature phase shift keyed (QPSK) known data in predetermined time frames to any receivers within range. The frames may propagate in a discontinuous transmission (DTX) mode. For voice traffic, transmission of user data occurs when the user speaks, but no data symbol transmission occurs when the user is silent. Similarly for packet data, the user data may be transmitted only when packets are ready to be sent. The frames are subdivided into sixteen equal time slots of 0.625 milliseconds each. Each time slot is further subdivided into equal symbol times. At a data rate of 32 KSPS, for example, each time slot includes twenty symbol times. Each frame includes pilot symbols as well as other control symbols such as transmit power control (TPC) symbols and rate information (RI) symbols. These control symbols include multiple bits otherwise known as chips to distinguish them from data bits. The chip transmission time (Tc), therefore, is equal to the symbol time rate (T) divided by the number of chips in the symbol (N).
Previous studies have shown that multiple transmit antennas may improve reception by increasing transmit diversity for narrow band communication systems. In their paper New Detection Schemes for Transmit Diversity with no Channel Estimation, Tarokh et al. describe such a transmit diversity scheme for a TDMA system. The same concept is described in A Simple Transmitter Diversity Technique for Wireless Communications by Alamouti. Tarokh et al. and Alamouti, however, fail to teach such a transmit diversity scheme for a WCDMA communication system.
Other studies have investigated open loop transmit diversity schemes such as orthogonal transmit diversity (OTD) and time switched time diversity (TSTD) for WCDMA systems. Both OTD and TSTD systems have similar performance. Both use multiple transmit antennas to provide some diversity against fading, particularly at low Doppler rates and when there are insufficient paths for the rake receiver. Both OTD and TSTD systems, however, fail to exploit the extra path diversity that is possible for open loop systems. For example, the OTD encoder circuit of FIG. 5 receives symbols S1, and S2 on lead 500 and produces output signals on leads 504 and 506 for transmission by first and second antennas, respectively. These transmitted signals are received by a despreader input circuit (not shown). The despreader circuit sums received chip signals over a respective symbol time to produce first and second output signals Rj1 and Rj2 on leads 620 and 622 as in equations [1-2], respectively.                               R          j          1                =                                            ∑                              i                =                0                                            N                -                1                                      ⁢                          xe2x80x83                        ⁢                                          r                j                            ⁡                              (                                  i                  +                                      τ                    j                                                  )                                              =                                                    α                j                1                            ⁢                              S                1                                      +                                          α                j                2                            ⁢                              S                2                                                                        [        1        ]                                          R          j          2                =                                            ∑                              i                =                N                                                              2                  ⁢                  N                                -                1                                      ⁢                          xe2x80x83                        ⁢                                          r                j                            ⁡                              (                                  i                  +                                      τ                    j                                                  )                                              =                                                    α                j                1                            ⁢                              S                1                                      -                                          α                j                2                            ⁢                              S                2                                                                        [        2        ]            
The OTD phase correction circuit of FIG. 6 receives the output signals Rj1 and Rj2 corresponding to the jth of L multiple signal paths. The phase correction circuit produces soft outputs or signal estimates {tilde over (S)}1 and {tilde over (S)}2 for symbols S1 and S2 at leads 616 and 618 as shown in equations [3-4], respectively.                                           S            ~                    1                =                                            ∑                              j                =                1                            L                        ⁢                          xe2x80x83                        ⁢                                          (                                                      R                    j                    1                                    +                                      R                    j                    2                                                  )                            ⁢                              α                j                                  1                  *                                                              =                                    ∑                              j                =                1                            L                        ⁢                          xe2x80x83                        ⁢                          2              ⁢                                                "LeftBracketingBar"                                      α                    j                    1                                    "RightBracketingBar"                                2                            ⁢                              S                1                                                                        [        3        ]                                                      S            ~                    2                =                                            ∑                              j                =                1                            L                        ⁢                          xe2x80x83                        ⁢                                          (                                                      R                    j                    1                                    -                                      R                    j                    2                                                  )                            ⁢                              α                j                                  2                  *                                                              =                                    ∑                              j                =                1                            L                        ⁢                          xe2x80x83                        ⁢                          2              ⁢                                                "LeftBracketingBar"                                      α                    j                    2                                    "RightBracketingBar"                                2                            ⁢                              S                2                                                                        [        4        ]            
Equations [3-4] show that the OTD method provides a single channel estimate xcex1 for each path j. A similar analysis for the TSTD system yields the same result. The OTD and TSTD methods, therefore, are limited to a path diversity of L. This path diversity limitation fails to exploit the extra path diversity that is possible for open loop systems as will be explained in detail.
Hosur et al. previously taught a new method for frame synchronization with space time transmit diversity (STTD) having a path diversity of 2L in U.S. Pat. application Ser. No. 09/195,942, filed Nov. 19, 1998, and incorporated herein by reference. Therein, Hosur et al. taught advantages of this increased diversity for WCDMA systems. Hosur et al. did not teach or suggest how this improved diversity might relate to Doppler frequency estimation.
Doppler frequency estimation is particularly critical in WCDMA systems where a mobile receiver may move with respect to a base station by car or high-speed train. Such motion may produce an apparent change of frequency or Doppler frequency shift of over 500 Hz. A reliable Doppler frequency estimate is important for Rayleigh fading parameter channel estimates, transmit power control (TPC) estimates and efficient use of downlink transmit antenna diversity such as STTD. Knowledge of the Doppler frequency is equally important for other channel estimate schemes such as iterative channel estimation (ICE) or weighted multi-slot averaging (WMSA). For example, knowledge of a Doppler frequency shift permits use of an optimal Weiner filter for channel estimates. If the Doppler frequency shift is unknown, the same filter must be used for a wide range of Doppler frequencies resulting in a degraded link margin. Use of an optimal filter is also highly advantageous in TPC estimation. Furthermore, downlink transmit antenna diversity operates differently with varying Doppler frequencies. At low Doppler frequencies, for example, a mobile system may send information to the base station indicating which antenna signal is stronger. At high Doppler frequencies, space-time code across transmit antennas may be used to achieve transmit diversity.
The foregoing problems are resolved by a circuit coupled to receive a sequence of signals comprising a multiplication circuit coupled to receive a first signal, a second signal and a complex conjugate of the first signal. The second signal follows the first signal in time. The multiplication circuit produces a first product sequence of the first signal and the complex conjugate and a second product sequence of the second signal and the complex conjugate. A summation circuit is coupled to receive the first product sequence and the second product sequence. The summation circuit produces a first sum of the first product sequence and a second sum of the second product sequence.
The present invention provides highly accurate estimates of Doppler frequency shift. The estimates are compatible with STTD and other diversity schemes.